In recent years there has been considerable interest in determining the contributions to embryonic and adult phenotype by the pronuclei and cytoplasm of mammalian embryos. For example, hairpin-tail (T.sup.hp) in mice is an allele of branchyury (T) situated on chromosome 17 (linkage group IX) of the mouse (Johnson, D. R., Genetics 76, 795-805 (1974)). It appears to differ from all previously described mammalian genes in that the phenotype and viability of the heterozygote depend upon the parent from which the T.sup.hp gene is inherited. Heterozygotes with a T.sup.hp /+ father are short tailed and generally viable. On the other hand, the majority of the heterozygotes whose T.sup.hp was derived from the egg die in the latter half of embryogenesis (gestational days 15-16) with a few exceptional T.sup.hp /+ embryos surviving until birth (Johnson, D. R. Genet. Res. 24, 207-213 (1975); Alton A. Doctoral Dissertation, Cornell U., N.Y. (1982)). Those T.sup.hp /+ progeny born, however, are cyanotic and die within 24 hours of parturition (Johnson, D. R. (1975), supra.). The T.sup.hp maternal lethal effect transmitted through the ovium could be inherited via the cytoplasm (oogenic defect) or via the female pronucleous (embryogenic defect). Thus, a method for determing whether this defect is transmitted by the pronucleus or cytomplasm of the egg would be most useful.
Another problem is that of breeding barriers. There are several different species of mice. Some can interbreed, while others cannot. The breeding barrier is caused by the plasma membrane and cytoplasm of the embryo. Accordingly, it would be desirable to provide a method whereby the breeding barrier could be overcome.
Another area of concern is the potential extinction of certain rare mammalian species, an example of which is the Siberian tiger. There are very few of this species remaining and their reproductive performance is poor. Accordingly it would be desirable if a method could be devised whereby such species could be preserved.
It would also be desirable to develop certain mammalian strains having desirable genetic characteristics provided by the pronucleus of one genotype and those provided by the cytoplasm of another genotype.
Nuclear trannsplantation methods might provide a viable solution to the foregoing problems. Nuclear transplantation studies in the amphibian embryo have provided valuable information about the possible restriction of nuclear potential during development (Danielli, J. R., et al, Int. Rev. Cytol. Suppl. 9 (1979)). Similar experiments in the mammalian embryo have been hindered by the small size of the embryo and its sensitivity to microsurgical manpulation. Although some success in nuclear transplantation in the mouse embryo by microsurgery has been reported (Illmensee K. et al, Cell 23 9-18 (1981)), it is believed to be due to the fact that the enucleation of the recipient cell in a small number of cases was incomplete enabling growth.